The invention relates to a projection television display tube having an evacuated envelope with a display window which has a display screen on its inside and in front of which a light-permeable second window is provided on its outside, a cooling liquid flowing through the space between the display window and the second window from at least one inlet aperture to at least one outlet aperture.
Such a display tube is known from Netherlands Patent Application No. 80.03.360 laid open to public inspection. By means of an electron beam a frame is written on the display screen which comprises at least one phosphor layer or a pattern of different phosphors. As a result of the electron bombardment the temperature of the phosphor increases so that the luminous efficiency of the display screen decreases ("thermal quenching"). This phenomenon occurs in particular in display tubes for projection television in which for maintaining the required high luminous densities the display screen is scanned by electron beams of high beam currents. At the same time the temperature of the display window increases and a temperature gradient is formed at the display window. This gradient causes a mechanical stress in the display window which consists, for example, of glass. At high electron beam current and consequently high thermal load this may lead to fracture of the display window. In order to reduce said mechanical stresses in the display window by temperature differences ("thermal stress") and to avoid the reduction of the luminous efficiency, it is known from the already mentioned Netherlands Patent Application No. 80.03.360 to cool the display window and the display screen connected thereto. The space between the display window and the second window filled with cooling liquid in a first described embodiment is surrounded on the top, at the bottom and laterally by a metal cooling member which serves as a spacing member and as a heat radiator. As a result of the rise in temperature of the display window the cooling liquid heated by the display window moves along the display window upwards and past the second window downwards as a result of which the thermal energy from the centre of the display window is also dissipated via the cooling member. At low load, for example smaller than 5 W, the thermal energy is dissipated to the second window substantially by conduction. At higher load the above-described liquid flow occurs with an associated additional cooling by the cooling member which, however, is little effective. Moreover, an embodiment is described in which the cooling liquid is subjected to cooling outside the space. For that purpose the cooling liquid is applied to the space from the top side of the space through pipes or hoses and through a cooling chamber to the lower side, namely by flow caused by temperature differences in the cooling liquid. A disadvantage of such a tube is that when the tube in a projector has to be replaced the cooling liquid must be removed and the hoses and pipes, respectively, must be disconnected from the display tube.
It is the object of the invention, starting from the above-described prior art, to provide a display tube having a more active cooling system so that an effective cooling is obtained at a power up to 40 W.
Another object of the invention is to provide a display tube having a substantially homogeneous temperature distribution of the display screen.
Still a further object of the invention is to provide a display tube having a cooling without additional pipes and individual heat exchangers.